Electric control device



Patented May 6, 1930 UNITED STATES PATENT OFFICE WARREN A HARRISON,

LABORATORIES, INCORPORATED, OF NEW YORK, N. 'Y.,

YORK

0]! ORANGE, NEW JERSEY, AQSIGNOR TO BELL TELEPHONE A CORPORATION OF NEWELECTRIC CONTROL DEVICE Application filed October 19, 1920. Serial No.142,573.

This invention relates to electrical control circuits and has for anobject to control the operationof a circuit by the use of extremelysmall currents.

Another object of the invention is to ener-- ture control systems inwhich a device for increasing or decreasing the current through anelectrical heater is operated by the openingor closing of an electricalcircuit through a temperature responsive device such as a mercury columnand its associated electrode. In order to prevent sparking between themercury column and the electrode, which would cause the deterioration ofthe contact and reduce the accuracy of its operation, it is desirablethat the current carried by the circuit throughv the mercur column andthe inductance of the circuit e made small.

In accordance with a feature of this invention, a relay is operated bythe space current of a three-electrode electric discharge device; animpedance, which may be a winding of the relay, is connected to thedevice in such a way that the voltage drop across its terminals producedby the space current flowing therein is impressed between the controlelectrode and the cathode; and the operation of the relay is controlledby a variable impedance connection from the control electrode to thecathode, by means of which the biasing potential is changed.

This invention not only provides an arrangement in which the operationof the relay may be controlled through a circuit which carries a verysmall current but also one which is readily adaptable to be energizedfrom a single source of alternating current. Thus, the cathode of thedischarge device may be supplied with heating current from one windingof an alternating current transformer 'while space current is suppliedfrom another winding, the discharge device functioning as its ownrectifier and the operating current for the relay being obtained fromthe plate circuit of the discharge device.

In one specific embodiment of the invention, the control circuit isemployed for regulating an electrical heater, the thermostat beingrequired to carry only a minute current limited by a high resistanceconnected in the grid circuit of the discharge device, and the controlcircuit and electrical heater both receiving current from a singlesource of alternating current.

In another specific embodiment of the invention,the variable impedanceconnection for controlling the biasing potential comprises a secondelectric dischar' e device, the impedance of which is control ed bymeans of electromagnetic Waves received from a distant radiotransmitting station.

Obviously, the feature of the latter embodiment, namely that ofoperating the control circuit through the use of a second dischargedevice, may be employed in any case in which it is desired to operatethe control circuit from an electrical source of low energy level.

The following is a description of the invention in its several aspectsso that it may be readily understood by those skilled in the art. F ig.-1 is a diagrammatic showing of the electric control device of theinvention.

Fig. 2 is a diagrammatic showin of the control device applied toareceiver ot electromagnetic waves, and i Fig. 3 is a diagrammaticshowing of the control device applied to a thermostat.

The control circuit shown in Fig. 1 employs a vacuum tube 10 having acathode 11, anode 12- and grid 13. The primary winding 14 of transformer15 is connected to a source of alternating current 16 and the secondarywinding 17 is connected to the cathode 11 of the vacuum tube. Oneterminal of another secondary winding 18 is connected to the anode 12while the other terminal of this winding is connected through a variableresistance '19 and the operating winding of a relay 20 to the mi -pointof secondary winding 17. A condenser 21 of large capacity is connectedbetween the mid-point of the transformerwinding17andtheterminalofthetransformer winding 18 which is connected'to the resistance'19. This terminal of Winding 18 is also connected toone terminal of a resistance 22,

preferably of several megohms, the other terminal of this resistancebeing connected to the grid 13. The poles of switch 23 are connected tothe grid-13 and the mid-point of the transformer winding 17,respectively.

In operation, the alternating potential supplied through transformerwinding 18 to the anode 12 of the vacuum tube 10, causes rectifiedcurrent to flow in a positive direction from the cathode 11 throughtransformer winding 17, the winding of relay 20, resistance 19,transformer 18, thence back to the anode 12. The condenser 21 serves toshunt out a large portion of the alternating current component of therectified current flowing in the anode circuit. The potential dropacross the resistance 19 and the relay winding 20 is impressed upon thegrid 13 through the resistance 22 for supplying a negative bias to thegrid, thereby limiting the current flowing in the anode circuit andthrough the winding of relay 20. The current through this re lay windingis adjusted by means of the resistance 19 to such a value that it isnornially insuificient to cause the operation of the relay. When theswitch 23 is closed, however, a low resistance connection is providedbetween the grid 13 and the cathode 11 of tube 10 thus changing thegridbiasing potential from a negative value to practically zero. Due to thischange in grid bias, the current flowing through the anode circuit isincreased to a value sufficient to cause the operation of relay 20. Byemploying a resistance 22 of large value the current flowing through thecontacts of switch 23 may be exceedingly small, for example, as low as afew micro-amperes, and since the impedance of the circuit through whichthis current flows has only a negligible reactance component,practically no sparking or heating occurs at the switch contacts. Thefact that the value of resistance 22 is high also prevents the relaywinding 20 from being short circuited by closing switch 23.

The control device shown in Fig. 1 may be adapted to numerous practicalapplications by substituting other variable impedance connections forthe switch 23. In Fig. 2 the input circuit of the device is shownconnected to the anode circuit of a vacuum tube detector 25 and isadapted to be operated by electromagnetic waves which reach the antenna24 and are transmitted to the grid of the detector tube through thetuned circuit 26. The other circuit elements are the same as those inFig. 1. e

As is well known, the current flowing in the anode circuit and theresistance measured between the anode and cathode of a vacuum tube varywith the potential difference between the control electrode and thecathode. When, for instance, the grid is biased by a negative potentialof sufficient magnitude with respect to the cathode, the anode currentI,,, for a given value of anode potential, may be reduced to practicallyzero or a low value which We may designate as I If an alternatingpotential having a peak voltage approximately equal to the grid biasingpotential is now impressed upon the grid circuit, the anode current Iwill vary in accordance with the frequency of the alternating potentialfrom zero to a value of I, which we may designate as I The values of thedirect current resistance 13,, measured between the anode and cathode ofthe tube corresponding to the values of I and I respectively, may bedesignated as r and 7", representing high and low values of resistance,respectively. The variations in 73,, corresponding to changes in I fromO to I will be from infinity to some finite value of several thousandohms.

The efi'ect of impressing the alternating potential upon the grid of thedetector tube 25, suitably biased as indicated above, is to change itsanode resistance from a value r n to a value which changes in accordancewith the frequency of the impressed voltage from m. to 1", The value of13,; is sufficiently low that the magnitude of the negative grid bias ofthe vacuum tube 10 is considerably reduced for half the time that thealternating voltage is impressed on the grid of the detector tube 25,thus increasing the R. M. S. value of the anode current in tube 10.Since the relay 20 is unable to respond to the rapid change in currentdue to variations of the grid bias of the vacuum tube 10, it remains inthe operated position during the whole period that the alternatingpotential is applied to the grid of vacuum tube 25.

In Fig. 3 the control circuit is shown applied to a thermostat forcontrolling an electric heater. The control circuit proper is the sameas that of Fig. 1. The arrangement employed to vary the cathodepotential applied to the grid of vacuum tube 10, consists of a mercurythermometer having an electrode 30 in contact with the mercury in thebulb of the thermometer and an electrode 31 with which the mercurycolumn makes contact when the temperature is sufliciently high. Thethermometer is immersed in a bath 32 the temperature of which is to becontrolled and which may be heated by the electric heater 33. Thecontacts of relay 20, in the anode circuit of vacuum tube 10, serve toshort-circuit the rheostat 34 and to operate either of the motors 36 or37, these motors serving to simultaneously revolve the contact arms 38and 39 of rheostats 34 and 35 through the differential gear 40.

In operation, when the temperature of the bath 32 is sufficiently highto bring the mercury column in contact with electrode 31, the grid biasof vacuum tube 10 is changed from a negative potential to practicallythe potential of the cathode. The anode current is consequentlyincreased sufliciently to cause the operation of relay 20 thus openingthe connection between armature 45 and contact 43 and closing theconnection between armature 45 and contact 42. At the same time theconnection between armature 44 and contact 41 is opened. For thiscondition the heating circuit may be traced from the upper terminal oftransformer winding 46 through the resistance of rheostat 34, contactarm 38 of this rheostat, the resistance of rheostat 35, contact arm 39of this rheostat, heater 33 and thence to the lower terminal of theWinding 46. Starting from the upper terminal of the power source 16, acircuit may be traced through motor 37, armature 45 of relay 20, contact42 back to the lower terminal of the power source 16, the motor 36 beingshortcircuited through armature 45 and contact 42 of relay 20. Whenmotor 37 is in operation the arms 38 and 39 of rheostats 34 and 35respectively, are revolved in a counter-clockwise direction, thusincreasing the resistance in series with heater 33 and reducing thecurrent therethrough. This operation is continuous until the temperatureof bath- 32 decreases suiiiciently to break the contact between themercury column and the electrode 31, thus restoring the negative bias tothe grid of vacuum tube 10 and causing the armatures 44 and 45 of relay20 to be restored to the position shown in the figure. As a result,rheostat 34 and motor 37 are short-circuited and a motor 36 is operatedthus causing the clockwise rotation of the arms 38 and 39 of rheostats34 and 35, respectively, and reducing the efiective resistance in theheating circuit. A graph of the heatin current plotted against timewould show a s owly decreasing current followed by an abrupt increase,then a slowly increasin current followed by an abrupt decrease. hen thesystem is in equilibrium the periods of increasing and decreasingcurrent are approximately equal.

When not in equilibrium, however, one of these periods is longer thanthe other so that the system is gradually brought to the equilibriumcondition.

One of the advantages of this type of control is that the resistance 22may have a sufficiently high value, several megohms for example, so thatthe current flowing in the circuit connected to the input of the controldevice is very small, for example, several microamperes. This togetherwith the fact that the input circuit is practically non-inductive,substantially eliminates sparking at the contact points in the shuntcircuit. The useful life and accuracy of a circuit closing means such asa mercury contact shown in Fig. 3 is, as a result considerablyincreased. A. second advantage is that the control device does notrequire the opening and closing of a circuit for its operation butfunctions satisfactorily by employing across its input a shunt circuitof variable resistance such as the anode circuit of a vacuum tubeemplo'yed as a detector of electromagnetic waves. The

device is operable, moreover, from a single source of alternatingcurrent power of any frequency.

lVhat is claimed is:

1. In combination, an electric discharge device having a cathode, ananode, and an impedance control element, a source of alternating currentfor supplying space current to said device, an impedance elementpermanently connected in series with said alternating current source inthe space current circuit of said device, connections from saidimpedance element to said control element including aresistance forimpressing a biasing potential thereon, and means for at times directlyconnecting said grid to said cathode independent of said resistance forcontrolling the space current of said device.

2. A combination according to the next preceding claim in which saidimpedance element comprises the winding of a relay to be operated by thespace current of the discharge device.

3. An electric control system comprising an electric discharge devicehaving a cathode, a grid and an anode, input and output circuitstherefor, a heat supply controlling device in said output circuitoperable upon variation in the anode current, means for normallymaintaining a potential difference between the grid and the cathode,temperature responsive means in said input circuit for varying saidpotential difference, and a common source of alternating current forsupplying energy to the cathode and the output circuits and for applyingpotential to the input circuit.

4. An electric control device comprising a vacuum tube having a grid, acathode and an anode, a switching device and asource of alternatingcurrent connected between said cathode and anode, a resistance connectedin said anode circuit, a connection from said resistance to said gridcircuit for maintaining a potential difference between said cathode andgrid, and a circuit closing means for connecting a shunt path acrosssaid id and cathode for reducing the potential ifference between saidgrid and cathode.

5. In combination an electric discharge device having a cathode, ananode and an im pedance control element, a source of alternating currentfor heating said cathode and for supplying alternating anode voltage tosaid device, an impedance element in the anode circuit of said electricdischarge device, a relay in the anode circuit of said electricdischarge device which normally carries insuiiicient current to causeits operation, a connec-,

tion from said impedance element to said impedance control element fornormally applying a uni-directional biasing potential to said controlelement, and means for removing said biasing potential to cause theoperation of said relay while maintaining current flow through saidimpedance.

6. In combination, an electric discharge device having a cathode, ananode and an impedance control element, a resistance, a relay, saidresistance and said relay being in the anode circuit of said electricdischarge device, a connection from said resistance to said impedancecontrol element for applying sufficient biasing potential thereto 50that the current fiowiiig through said relay is insufiicient to causeits operation, and means for removing said biasing potential to causethe relay to operate while maintaining current flow through saidresistance.

7 A temperature control system, comprising a vacuum tube having a grid,a cathode and an anode, a relay in said anode circuit operable uponvariation of current therein, an electric heater, electrically actuatedmeans for varying the current through said heater, means for normallysupplying a biasing po tential to said grid, temperature responsivemeans for altering said biasing potential, and a common source ofalternating current for supplying the cathode and anode current and thegrid biasing potential of said vacuum tube, the current through saidheater, and the current for operating the means whereby the heatercurrent is varied.

8. In combination, an electric discharge tube having an electronemitting cathode, an anode, and an impedance control element, a sourceof alternating current, connections from said source for maintaining analternating potential between the cathode and the anode, a deviceoperated by the space current of said tube, means including an impedancein the cathode-anode circuit for deriving a potential from said sourcefor normally maintaining a uni-directional potential difference betweenthe control element and the cathode such as to prevent the operation ofsaid device, and means for shifting said potential difierence to a valuepermitting operation of said device whilemaintaining current flowthrough said impedance.

9. An electric relay comprising a vacuum tube having a cathode, grid andanode, a source of alternating current supplying said cathode and anode,an impedance in said anode circuit, a source of uni-directional potentia-l for said grid obtained from the potential drop across saidimpedance, said impedance bein common to the grid and anode circuits ofsaid tube, and a shunt across said impedance for effectively eliminatingsaid impedance from the grid circuit of said tube while effectivelymaintaining it in said anode circuit.

10. An electric relay for controlling a large current with acomparatively small current comprising a vacuum tube having a cathode,grid and anode, an impedance, and a path containing a contact pointshunting said im-

